This invention relates to novel N-oxyl compounds, mixtures of such N-oxyl compounds, a process for preparing them, and a method for inhibiting the polymerization of vinyl monomers, such as xcex1,xcex2-unsaturated carboxylic acids and esters thereof, by using them.
xcex1,xcex2-Unsaturated carboxylic acids and esters thereof, which are typical of vinyl monomers, are compounds having a wide variety of uses as monomers for the production of various polymers and copolymers. However, since they have the property of being very easily polymerized, polymerization troubles due to causative factors such as heat and light may frequently occur during the course of their production, storage, transport and the like. Especially in the production of xcex1,xcex2-unsaturated carboxylic acids and esters thereof, they are exposed to high temperatures above 100xc2x0 C. and hence tend to cause the trouble of forming a popcorn polymer in the liquid and vapor phases.
In order to prevent the above-described polymerization troubles occurring in xcex1,xcex2-unsaturated carboxylic acids and esters thereof, attempts have been made to inhibit their polymerization by using various polymerization inhibitors. Such polymerization inhibitors include, for example, heteroaromatic compounds such as phenothiazine; phenols such as hydroquinone and hydroquinone monomethyl ether; and aromatic amines such as N,Nxe2x80x2-di-2-naphthyl-p-phenylenediamine [Nonflex F (trade name), manufactured by Seiko Chemical Co., Ltd.; hereinafter abbreviated as xe2x80x9cAFxe2x80x9d], N-phenyl-Nxe2x80x2-(1,3-dimethylbutyl)-p-phenylenediamine [Nocrak 6C (trade name), manufactured by Ouchishinko Chemical Industrial Co., Ltd.; hereinafter abbreviated as xe2x80x9c6Cxe2x80x9d] and N,Nxe2x80x2-diphenyl-p-phenylenediamine [Nonflex H (trade name), manufactured by Seiko Chemical Co., Ltd.]. Moreover, Japanese Patent Laid-Open Nos. 320205/""93 and 320217/""93 disclose a method using an N-oxyl compound of the following formula (2) alone or in combination with other polymerization inhibitors. 
wherein R8 represents hydrogen, an alkyl group or an acyl group.
However, under high-temperature conditions employed for the purpose of reaction and purification by distillation in processes for the production of xcex1,xcex2-unsaturated carboxylic acids and esters thereof, conventional polymerization inhibitors fail to exhibit a satisfactory polymerization-inhibiting effect. Consequently, these polymerization inhibitors need to be added in large amounts and hence have a problem from the viewpoint of industrial use. Moreover, although the N-oxyl compounds of formula (2) in which R8 is hydrogen or an alkyl group have an excellent polymerization-inhibiting ability even when used in small amounts, the actually known compounds of this type are limited to those in which R8 is hydrogen, methyl or ethyl. These compounds have relatively low boiling points or sublimation temperatures which are close to the boiling points of xcex1,xcex2-unsaturated carboxylic acids and esters thereof, and hence involve the problem that, during the purification of xcex1,xcex2-unsaturated carboxylic acids and esters thereof by distillation, they are distilled out together with the product to cause a coloration thereof. On the other hand, the N-oxyl compounds of formula (2) in which R8 is an acyl group have sufficiently high boiling points or sublimation temperatures. However, during use, they may be converted to compounds having low boiling points or sublimation temperatures and may eventually be distilled out together with the product. As an example of this phenomenon, when an N-oxyl compound of formula (2) is used as a polymerization inhibitor for the ester exchange reaction between methyl (meth)acrylate and an alcohol, the polymerization inhibitor itself may undergo an ester exchange reaction to form a compound containing a (meth)acryloyl group as R8 and hence having a low boiling point or sublimation temperature, and may eventually be distilled out together with the product to cause a coloration thereof.
An object of the present invention is to provide novel N-oxyl compounds and a process for preparing them.
Another object of the present invention is to provide a polymerization inhibition method which, when employed in processes for the production of vinyl monomers such as xcex1, xcex2-unsaturated carboxylic acids and esters thereof, causes little coloration of the product and does not require a large amount of polymerization inhibitor.
That is, the present invention relates to an N-oxyl compound of the following formula (1). 
wherein n is an integer of 1 to 18; R1 and R2 are each hydrogen or methyl, but at least one of them is hydrogen; R3, R4, R5 and R6 are each a straight-chain or branched alkyl group; and R7 is hydrogen or (meth)acryloyl.
Moreover, the present invention also relates to a process for the preparation of an N-oxyl compound of formula (1) which comprises the steps of effecting the addition of ethylene oxide and/or propylene oxide to a 4-hydroxy-2,2,6,6-tetraalkylpiperidine-N-oxyl, and optionally esterifying the resulting product with a (meth)acryloyl-containing compound.
Furthermore, the present invention also relates to a polymerization inhibition method wherein an N-oxyl compound of formula (1) or a mixture of such N-oxyl compounds is used as a polymerization inhibitor for vinyl monomers.